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Rebuilding the Hammarlund SP-600

~ General Information about the SP-600 ~

~ The Molded Capacitor Problem ~

~ Receiver Disassembly and Capacitor Replacement ~

~ Rebuilding Suggestions and Mechanical Restoration ~

 ~ Thorough Cleaning and RF/IF Alignment ~

~ Performance Expectations ~


by: Henry Rogers - WA7YBS - WHRM

 


The 1953 Hammarlund SP-600 JX-21

The Hammarlund SP-600 is certainly a favorite receiver of many vintage radio operators. The SP-600's massive proportions, super-smooth tuning and great performance usually result in the receiver assuming a prominent position in the ham station landscape. Unfortunately, all early production SP-600 used leakage-prone molded capacitors in the circuitry. For top-performance and reliability, these early receivers must be rebuilt. Unfortunately, over half of the fifty capacitors that need to be replaced are hidden in various assemblies within the receiver. These assemblies will have to be removed for molded capacitor replacement. This web-article provides detailed information (with plenty of photographs) on the hidden capacitor locations, the rework necessary for the various assemblies and other important rebuilding information.  Henry Rogers Dec. 11, 2009

The Hammarlund SP-600 Series of Receivers

General Information about the SP-600

Hammarlund released an advertisement in 1948 that introduced their newest receiver, the SPC-600X. The receiver was to sell for $395 but, as Hammarlund had done before, this advertisement was really just to let hams and interested commercial users know what was going to be available in the future. No SPC-600X receivers were ever created other than maybe a prototype or two. It was over a year later, in 1950, that Hammarlund introduced the SP-600 Series and the selling price had dramatically risen to nearly $1000. This new SP-600 was primarily intended for the military and commercial user market. It was a very popular receiver and many thousands were built, especially for military applications. It's likely that some of the SP-600 design criteria actually came from the U.S. Army Signal Corps, especially the JX crystal oscillator concept. The use of the selectable crystal oscillator is seen on some "customized" SP-200 WWII Super Pro receivers and was even designated as an "Improvement Kit" with the part number MC-531 from the Signal Corps. Sometimes these receivers were "custom" rebuilds done by various engineering companies for the Signal Corps. These "custom" SP-200 Super Pro receivers usually date from 1947 up into the 1950s. It's likely that the turret band changing was also a specification from the Signal Corps and this is shown by the Hallicrafters' R-274 receiver, built for the Signal Corps and sharing many similarities to the SP-600, including the turret band switching concept. So, although Hammarlund advertising appears to ignore the Signal Corps input, there is just too much evidence available that shows much of Hammarlund's design concepts for the SP-600 actually came from the Signal Corps.

Most SP-600 versions were built throughout the 1950s but the SP-600 continued to be produced in smaller numbers up into the early 1970s. The standard SP-600 tunes from .54 to 54MC in six bands. A "J" suffix indicates JAN parts were used in the construction and an "X" suffix indicates a switchable crystal LO. Hammarlund also offered a "JLX" version with 100-400KC substituted for the .54-1.35MC band and a "VLF" version that covered 10-540KC. Hammarlund made over 40 variations of the SP-600 that were each assigned a numerical suffix which identified the particular circuit, mechanical changes or sometimes the end-user. The last in the "time-line" was the model variation SP-600 JX-21A from 1969-1972 which utilized a product detector circuit, two additional tubes and some other changes to make it "compatible" with SSB operations.

Most versions use a 20 tube double conversion superheterodyne circuit with a rotating turret bandswitch. The receivers also feature enormous proportions, robust construction and oversize controls - along with a super-smooth tuning system that only adds to the enjoyment of operating these fine receivers. Double conversion is switched in above 7.4MC and uses a 3.5MC crystal-controlled conversion oscillator. Though the SP-600 has two dials, it has no bandspread - the right side dial is a logging scale allowing precise retuning of desired stations. On the left is the main tuning dial and the mechanically articulated dial pointer that indicates which tuning scale is in use (along with the small window between the dials that shows which tuning range is selected.) The tuning arrangement was an up-dated version of the "Continuous Bandspread" system introduced in RCA's AR-88 series receivers in the 1940s. The frequency readout accuracy is vague which is why a precise logging scale system is incorporated into the SP-600 design. The Carrier/Audio Level meter is not illuminated and a switch is provided to indicate either carrier level or audio output. Most (but not all) SP-600 model numbers usually will have a suffix with "J" or "JX" followed by a numeral. As mentioned above, suffix "J" indicated that, as much as possible, military level components and construction were used. Suffix "X" indicated that a switchable six-position, fixed-frequency crystal-controlled oscillator was installed that allowed the user to install HC-6/U type crystals for specific desired LO frequencies. The user could switch to any of the crystal LO frequencies for increased stability for that particular frequency - however, the receiver still has to be "tuned" to the desired frequency for the RF and Mixer stages to be in tune. The number following the suffix generally indicates specific features for that version, e.g., contract or end user, circuit changes, etc., with the number ranges being more or less chronological for introduction until the last of production. Though the numbers assigned are chronological for their introduction, many of the numbered versions were produced over a relatively long period of time that saw many engineering changes incorporated into the receivers. As an example, early 1953 JX-21 receivers are different from the 1956 USAF documented JX-21 and both of these versions are very different from the JX-21A of the late sixties. Many SP-600 receivers were set-up for diversity operation and the standard diversity model was the JX-17 version. This version was produced in large numbers and can be easily spotted by observing that it has two extra controls and uses three red colored knobs. The SP-600 Audio output is about 2 watts from a single 6V6 using a balanced split-winding audio output transformer for 600 ohms Z. A rebuilt SP-600 will provide an impressive "communications grade" type of audio reproduction.
Shown in the photos left and right are two advertisements for the SP-600. On the left is the initial 1948 ad for the Series 600 "Super-Pro," the SPC-600X. This was a "teaser" ad to show what Hammarlund was working on. It's possible that a prototype or two may have been made but the SPC-600X was never released as a sellable product. To the right is an ad for the SP-600 from 1963. By this time, Hammarlund was owned by Giannini Scientific Company and the products were built at Mars Hill, North Carolina. Note that the 1963 ad promotes the SP-600's wide frequency range (although you would have to buy two receivers to have the advertised coverage.) Also note that by 1963, it appears that only four SP-600 versions are being produced.

The influence of the earlier Super Pro receivers on the SP-600 design and construction is obvious. In 1931, Hammarlund sky-rocketed to the top of communications receiver technology with the introduction of their Comet Pro receiver, the first successful shortwave superheterodyne receiver. Hammarlund jumped to the top again with the introduction of the Super Pro in 1936. The Super Pro remained basically unchanged from 1939 up to the late forties and, by that time, it was apparent to most users that the SP-400 Super Pro was just a repackaged SP-200. Hammarlund, once again, had to come up with a tremendously innovative receiver and, in 1950, they once again "hit it out of the park" with the SP-600. The new receiver became a favorite of both military and commercial users. Nowadays, many rebuilt SP-600 receivers have found new homes in vintage AM ham stations around the world. Admired for its massive proportions, ultra-smooth tuning, great sensitivity and sheer dominance of the ham station landscape, SP-600 receivers are sought-after, rebuilt, restored and then finally, once again, put to work.

The Molded Capacitor Problem

Why You Should Rebuild Your SP-600 Receiver - The Hammarlund SP-600 series of receivers were built for commercial-military users who required a receiver designed for continuous use with maximum performance capabilities. However, that was over 50 years ago! Nowadays, all SP-600 receivers built in the early fifties to the mid-fifties, that have not been rebuilt at sometime in the past, will certainly need all of the leakage-prone molded tubular capacitors replaced. The SP-600 receiver circuit seems to be able to keep on functioning even with many leaky capacitors. This has caused many operators to continue to use their SP-600 thinking everything is okay, BUT the receiver isn't working to specifications and sooner or later (likely sooner) there will be a failure that could cause serious damage to the receiver. On that note, the leaky capacitors not only cause performance issues but are also likely to cause failure of other components in the receiver circuit. It is quite common to find several burned resistors in any early SP-600 that has not been rebuilt but was operated long hours with leaky capacitors. The burned resistors are due to the excessive leakage current that the molded capacitors develop after aging a decade or two. Two types of molded capacitors are found in early SP-600 receivers. Sprague manufactured the "Black Beauty" - a black plastic encapsulated, color-coded capacitor and Cornell-Dubilier supplied a gray plastic body, color-coded molded capacitor. Both types were used interchangeably by Hammarlund and both types exhibit high current leakage today. The molded capacitors were only used in early production SP-600 receivers - from 1950 up to around 1957. After that, more reliable ceramic disk capacitors were used. However, the largest production runs of SP-600 receivers date from 1950 up to about 1958 with somewhat smaller production runs until the early sixties. Thereafter, production levels were fairly low. Consequently, you are most likely to find an early production SP-600 unless you specifically look for the later versions (and they are certainly the more expensive receivers.) This article covers all of the normal problems encountered when rebuilding the early SP-600 receivers.

photo left: Top view of the 1953 SP-600 JX-21 chassis.

Some Preliminary Rebuilding Information

Difficulty, Experience and Workmanship - Rebuilding an SP-600 receiver is daunting. But it is a project that is well within the capabilities of most communications receiver restorers. You will have to perform extensive disassembly of the receiver and some of its assemblies in order to access all of the parts that need to be replaced (and others that need testing.) To successfully complete an SP-600 rebuild you should be experienced with complete mechanical disassembly and reassembly of receivers. You must be able to perform quality component removal and installation with a high quality soldering technique using first-class soldering equipment and real SnPb solder. You must be able to keep track of your rework on several different kinds of assemblies. You should have experience in troubleshooting and aligning communications receivers. That being said, when performing the actual rework the most important thing to keep in mind is - don't be in a hurry and be thorough in your inspection and rework. Even for experienced technicians, rebuilding an SP-600 is time consuming and exacting work but hurrying through this kind of job will just lead to mistakes and shoddy workmanship. Rebuilding the RF Platform is tedious work and its band switching contacts can be easily damaged if that unit is not handled carefully while it is out of the receiver. All of the turret modules will have to be removed but this is easier than it sounds. Again, these small units can be damaged by careless handling - be careful. It can be helpful to take notes on how the removed units are wired back into the circuit although there are excellent wiring drawings in the SP-600 manuals. Since the entire receiver is thoroughly disassembled to perform the rebuild a complete RF/IF alignment will be required. How well your rebuilt SP-600 operates will depend on your technical skills, your quality of workmanship, the type of soldering equipment you use and your rebuilding experience. The quality of your alignment test gear is also very important.

The Caveat - Though most SP-600 receivers look pretty much the same on the exterior, inside there are several differences between the many variations with different value components, additional parts or lack of parts being commonly encountered. The receivers shown in this article are two different versions of the early variety. The SP-600 JX-21 has the switchable crystal controlled oscillator and all of the modifications to incorporate the "JX" option. This is an early version (1953) JX-21, so it is somewhat different than the later 1956 USAF versions and completely different from the 1972 JX-21A version. The other receiver is the SP-600-25C. This receiver has the 25-60 cycle power transformer and lacks the switchable crystal controlled oscillator. There are several minor differences between these two types of SP-600 receivers and certainly many differences with the other 38 or so variations of the SP-600. So, if your SP-600 doesn't exactly conform to what is stated here, don't be too surprised - I won't be either.

 Manuals and Schematics - There are about 40 different versions of the SP-600 but the receivers can be roughly divided between early versions and later versions. This division is not based on the number suffix but rather when the particular receiver was manufactured. There aren't manuals for every single numbered version but it's fairly easy to find a manual and schematic that is proper for your particular version.

The Military came out with several different manuals for many of the variations, so it's possible to find both Signal Corps TMs (TM-11-851) and several types of USAF manuals. However, with some of the military manuals you will find there are earlier manufactured versions of the receiver than what is covered in the manual. This is particularly true of the JX-21 USAF manual which is dated 1956 and is for the later versions of the JX-21. The JX-21 was first built in early 1953, which means the 1956 USAF manual will have several differences from the early JX-21 receivers. This is true for almost all SP-600 receivers versus the available manuals. It is more important to use the manual that was issued closest to the date of manufacture of the receiver rather than going by the particular numerical version of the receiver. In other words - not all specifically numbered versions of the SP-600 were identically built.

With the early versions of the schematic as in Issue 1 and 2 of the Hammarlund manuals no component values are on the schematic. You'll have to use the parts list as a cross-reference to determine the correct value of the component shown on the schematic. Later schematics, from Issue 3 up, do have the component values on the schematic.

BAMA (Boat Anchor Manual Archive) has about four of the six issues of the Hammarlund manual online plus other SP-600 information. Generally, Hammarlund Issue 1 and 2 are for the earliest versions while Issue 3 is sort of a transition version. Issues  4, 5 and 6 are for the later versions. This is generally the case and it's best to down load all of the versions available to see which is closest to your particular SP-600.

There are some dealers doing business on eBay that sell repro manuals if you are interested in having hard copies of the Air Force issues and of the Signal Corps TM (around 200 pages, so it's expensive.) If your SP-600 was modified or has missing components you will absolutely need to use the manual and schematic that is correct for your receiver. Though it is possible to work through the SP-600 with general information from almost any manual, if the receiver has been compromised with modifications, you'll need accurate documentation to return the receiver to its original configuration.


photo above:  How the typical SP-600-JX receiver looks before restoration. Lots of dirt and corrosion on this one.


Photo above: A look under the chassis shows a lot of molded capacitors (even some replacement caps that are molded types.) Other serious problems are surely lurking inside this typical unrestored  SP-600 JX.

What Needs to be Taken Apart? - Here is the list of assemblies that need to be removed from the receiver in order to gain access inside the assembly to replace molded capacitors. Quantities shown can vary depending on which variation is being worked on. The quantity shown is for general information - your receiver may be different.

1. RF Platform - 20 molded capacitors - all are .01uf - 400wvdc

2. Turret Bandswitch - 6 molded capacitors - all are .01uf - 400wvdc - only the 1RF, 2RF and Mixer modules for the .54-1.35mc band and the 1.35-3.45mc band have molded capacitors that need to be replaced.

3. T-1 455kc/3.559mc Mixer Transformer - 1 molded capacitor. This unit is mounted to the side of the RF box next to the filter chokes. Inside the unit is one .01uf - 400wvdc molded capacitor.

4. Crystal Conversion Oscillator - 3 molded capacitors. This unit is mounted under the chassis. All capacitors inside are .022uf - 400wvdc.

5. IF Transformers - 2 molded capacitors inside the Conversion IF transformer, value is .022uf - 400wvdc. 1 molded capacitor inside two of the 455kc IF transformers, value is .022uf - 400wvdc. The Crystal Filter/IF transformer doesn't have molded capacitors inside.

6. Switchable Crystal Controlled Local Oscillator Assembly ("X" Option) - 2 molded capacitors inside this unit. Both are .01uf  - 400wvdc.

Additionally, most receivers will have two tub units that are double .05uf at 600wvdc capacitors. There is also a .05uf  tubular and a .25uf tubular (Sprague built units usually) that are mounted on standoffs on the front apron of the chassis. The usual procedure is to replace these with individual components when doing the rebuild. All of the other under-the-chassis molded capacitors are easy to access and replace. All total, the amount of molded capacitors that need to be replaced will exceed 50.

Components - When Hammarlund eliminated the molded capacitors from production they went to ceramic disk capacitors. That choice as new replacements would be fine, as would SBE Orange Drop or metalized film "Yellow-Jacket" types of replacement capacitors. The choice is not critical since any of these types of capacitors are at least a 1000 times better than the original molded capacitors.

Where to Start? - It might be a good idea to first get the receiver somewhat operational before starting disassembly. This will assure you that you won't encounter any really serious problems after you have gone through all of the work entailing the rebuild. Usually, you'll end up replacing a couple of capacitors and maybe a burned resistor to get the SP-600 running enough that you can check out all of the control functions to make sure the receiver is basically functional. Common sense should guide you in deciding whether or not to get the receiver entirely operational before starting the rework. When you are satisfied that nothing is seriously amiss with the receiver, you can begin disassembly which will require removing the front panel, the side panels, the tuning condenser cover and the bottom cover on the band switching turret. You can also remove the shield covers on all of the IF transformers, the shield cover on the BFO (to check parts) and the shield cover on the 3.5mc Crystal Conversion Oscillator. Everything will then be accessible for capacitor replacement and thoroughly checking all of the receiver circuitry.

Rebuilding the SP-600 Assemblies

The RF Platform

This is the most difficult assembly to remove, rebuild and then re-install. The bandswitch must be placed in a position exactly in-between any of the "lock-in" band select positions. This allows the short contact pins from the RF units in the turret to not be engaged into the contact fingers of the RF Platform. Remove the top cover over the tuning condenser and unsolder ALL of the connections to the tuning condenser. There are 16 connections that need to be de-soldered here. Then remove the top cover from T-1 and unsolder the wires that are connected to the terminals there. There are from five to seven wires that need to be unsoldered depending on which version is being worked on. Next remove the four mounting screws on the top of the RF Platform. Now, carefully lift the RF Platform up while guiding the T-1 wire cable as they pass through the opening to T-1. With the RF Platform out of the receiver, you'll be able to see 20 molded caps that need to be replaced. Be sure to watch the alignment holes versus the placement of the new capacitors. You want to have clearance from the alignment holes down through the holes in the ceramic mounts and down to the adjustments on the RF units of the turret. Make sure your placement of the new capacitors doesn't block alignment tool access. Try to mount the new capacitors in exactly the same position and same orientation as the originals. Check the contact fingers for corrosion and clean with De-Oxit applied with a Q-tip - nothing abrasive. Don't move the bandswitch control on the receiver until you have reinstalled the RF Platform. Re-installation requires that the wire cable be guided through the access hole to T-1 as you lower the RF Platform into place. Reconnect the wires to the T-1 terminal board and reconnect all 16 connections to the tuning condenser. Replace the mounting screws. Check for proper operation of the bandswitch-turret and smooth engagement of the contact pins of the turret into the RF Platform contacts.

photo above: The RF Platform showing the original Cornell-Dubilier molded capacitors. (SP-600-25C)

photo above:  Close-up of the broken contact finger on the RF Platform, 1st RF stage, Antenna input. These contact fingers are very fragile at the points where they enter into the ceramic mounts. The right-angle bends tend to be weak points.  Hammarlund soldered the bottom lugs on the contact fingers to lock them in place and to prevent "rocking" as the turret pins are engaged.  (SP-600 JX-21)

More on the RF Platform Rebuild - Often times you will find that the RF Platform was not carefully handled during a repair job that was performed in the past. Reference the photo to the left and note that the second contact finger in the front row is broken. This is the 1RF section and the Antenna Input connects to this contact finger. This is an old break and was discovered when I removed the RF Platform from the SP-600 JX-21 receiver during its rebuild. This particular receiver showed evidence that it had been worked on many times during the past by a technicians that were careless in their workmanship. The fact that the contact finger was left unrepaired seems to support this opinion. Repairing this broken contact finger is possible because only five contact fingers are used in the Local Oscillator, Mixer and 2RF sections but that Hammarlund made all of the ceramic mounts the same - with six contact fingers. The Mixer and the 2RF sections use the extra contact finger as a tie point for components but the LO section will have one unused contact finger present. It is possible to remove the unused contact finger from the LO section and then reinstall it to replace this broken one in the 1RF section. The 1RF section on bands 1 and 2 is the only place where all six contact fingers are used.

Removing the contact finger requires careful workmanship. You must use solder wick and remove absolutely all of the solder from the contact finger lugs. Next, insert an 18ga. wire into the "T" slot in the ceramic and push gently down on the tang that holds the contact finger in the ceramic. Once the tang is down, that side of the contact finger can be pushed slightly up. Insert the 18ga. wire in the other "T" slot and push down on that tang, then that side can be pushed slightly up. Once the two tangs are released the contact finger can be removed from the top of the ceramic mount. These contact fingers are very fragile and will break easily, so go slowly and carefully, then successful extraction is fairly easy. To install the good contact finger just push it into place in the slots and the tangs will lock. Then solder the bottom lugs to lock in place.

One can also note from the close-up photo to the left that there is some corrosion on the fingers and also a lot of dirt or "tuner grease." This should all be cleaned with denatured alcohol and a Q-tip. Stubborn areas may require a brass brush to remove the corrosion. When everything is clean then go over the contact fingers with De-Oxit D-5, which contains a corrosion cleaner along with a lubricant that works fine for these kind of contacts.

Careful, meticulous rework to the RF Platform is essential to regain the superior performance that the SP-600 is capable of.

A Thorough Inspection of the RF Platform Reveals More Problems - It is possible that some of the "leaky" original capacitors in the RF Platform circuit will have compromised some of the associated resistors in the circuit. Thoroughly inspect the RF Platform carefully for burned resistors and measure each of the resistor values just to be sure. As an illustration of just what might be found, note the photo to the right. This is the RF Platform from the SP-600-25C receiver. Note that R-15 is burned. This is a 510 ohm resistor that now measures 30 ohms due to the overheating that took place when in operation with the "leaky" original capacitor, C-43. It can been seen in the photo that the green-brown-brown color code has changed due to the heat - especially the second digit designator band which now appears flat, pale tan rather than brown. This suspicious appearance prompted physical measurement of the resistor value which showed that the value had changed from 510 ohms down to 30 ohms. Naturally, this resistor will have to be replaced along with all of the original tubular capacitors.

Another interesting observation is that R-16 is missing from the circuit. This 1K ohm resistor is the plate load for the 2RF amplifier. Looking at the photograph to the right, R-16 should be connected to the terminal to the rear that R-15 is also connected to. It can be seen that no resistor is mounted there. During the initial check out of the receiver, operation appeared normal (for an original condition receiver.) So one can conclude that even though the 2RF amplifier was not operating to its full capabilities, it did pass enough of the signal through that stage that the defect was not noticed in the initial operation check out. This shows that a thorough physical inspection of the RF Platform is necessary to find all of the defects that might be present. The Connection Diagram for the "Tuning Unit" (RF Platform) is extremely helpful for the inspection.  

photo right: Close-up of the Mixer section of the RF Platform which is where the plate circuitry is for the RF2 amplifier. Note the discoloration of R-15 and the missing R-16. (SP-600-25C)



photo above:  The band switching turret as viewed from the underside of the chassis with the shield-cover removed.  Note the curved retaining clips on each end of the RF modules.  (SP-600 JX-21)

The Bandswitching Turret

This is a really great piece of engineering. As the bandswitch control on the front panel is turned, the turret rotates and places into an engagement position the contact pins of four front-end modules that make up the 1RF, 2RF, Mixer and LO sections of the front-end for each tuning range of the receiver. Each front-end module is built onto a ceramic mount that has the tunable coil, the trimmer capacitor and a couple of resistors and fixed capacitors that make up each circuit. The connections are to short pins on the back of the ceramic mount that engage the RF Platform fingers when the turret is rotated into position. Each front-end module can easily be removed from the turret since they are only held in their mounted position by two arched clips.
 

Six of the RF modules use molded capacitors that will need to be replaced. All of the others have mica caps installed. The .54mc-1.35mc section will require replacement of a capacitor in the 1RF, 2RF and Mixer modules and the same units require a capacitor replacement in the 1.35mc-3.45mc band. Each module has one molded capacitor of .01uf at 400wvdc. While you are working on the turret, it's a good idea to check the resistors along with the other components installed in all of the front-end modules. The Mixer sections especially seem to sometimes have burned resistors but they do turn up in the other modules also. Once and a while you'll find someone has inadvertently injected high level RF into the receiver antenna input and burned the 1RF coil. Check the pins of each RF units for corrosion and cleanliness. Don't use anything abrasive unless corrosion is present, then you might have to use a brass brush and denatured alcohol to clean. Follow with De-Oxit applied with a Q-tip.

Be sure to check the tightness on all of the tensioners that are on the inductance adjustments. These are circular metal disks with slots along the edge that are threaded onto the inductance threaded shaft. These tensioners provide a spring load on the inductance adjustment so it will stay set. Sometimes these tensioners loosen and when that happens you'll find that the inductance adjustments don't seem to hold adjustment when doing your alignment. Check them - they should be a little more than snug but not over-tight. Too tight and you'll have difficulty adjusting the inductance with the alignment tool. The tensioner should be just tight enough to hold its position and not move when the inductance shaft is adjusted.

 

NOTE: Early modules will have different value resistors compared to later ones.

More on the Front-end RF Modules - It's probably a good idea to remove each one of the modules to closely check it over. Though you can look through between the modules and see most of the components, you can't see everything. As an example, look at the 2RF module for the 3.45mc to 7.4mc band shown in the photo to the right. The resistor is burned in two. It isn't really visible when the module is in place and by looking between the modules. It was found because each module was removed for close inspection. Besides the resistors, you also should look at the trimmer capacitor and check for bent plates and check the mica capacitors for cracks or other problems. The burned resistors are generally caused by leaky molded capacitors located in the RF Platform. Many times a rebuilder will recap the RF Platform and the six modules that have molded caps but not check each of the RF modules for damage. The end result is missing problems such as the one shown to the right. It pays to be thorough.

 

Photo left:  RF front-end module with the original C-D molded capacitor installed (SP-600-25C)
 


photo above: 2RF module (3.45-7.4mc) with burned 510 ohm resistor (SP-600 JX-21)

The IF Transformers

There are molded capacitors located in each of the IF transformers with the exception of T-3, the crystal filter/IF transformer assembly.  IF transformers T4 and T5 have one .022uf 400wvdc molded capacitor inside that will need to be replaced. T-2 has two .022uf 400wvdc molded capacitors inside that will need to be replaced. The IF transformer shields are held in place by four 6-32 screws that mount the shield to the chassis by way of right angle brackets on the shields. Removing the screws and the shield allows you access inside of the IF transformer. There is a blade spring that provides a load on the IF adjustment screws. Be sure to keep track of the spring and replace it when you re-install the shield. The molded capacitors are very easy to replace in all of the IF transformers. Be sure to check the value of the carbon resistors that are located inside the IF transformer shields since any leaky capacitors may have compromised the resistor value.

NOTE: Be aware that the IF transformers change slightly in the later versions of the SP-600 and will have slightly different connections to the terminals under the chassis.

photo right: The IF section of the SP-600 with the shields removed allowing access to the molded capacitors. The nearest IF transformer has already had the new SBE "Orange Drop" replacement capacitor installed. Note here that the 2200 ohm resistor is 5% - I had to replace the burned up original 10% resistor because of the leaky original capacitor (SP-600-25C.)


photo above: T-1 455kc/3.955mc Mixer Transformer with the shield removed showing the C-D molded capacitor located inside. This unit requires the removal of a couple of other adjacent assemblies just to gain access to replace the one molded capacitor located inside.  (SP-600-25C.)


 

T-1 455kc and 3.955mc Mixer Transformer

This is the small assembly that is mounted on the right side of the RF tuning box near the power supply filter chokes. It is held to the RF box by six screws that are difficult to access in the  non-"X" receivers because of their proximity to the power supply filter chokes and impossible to access in the receivers with the "X" option. In "X" optioned receivers, the Crystal Controlled Oscillator unit has to be removed first. Unsolder the four wires that connect to the Filter Capacitor/Filter Choke mounting assembly, remove the four screws and dismount the assembly. You don't have to unsolder the wires that connect to the Crystal Controlled Oscillator unless you are going to remove it for rebuilt, then unsolder them. Unscrew the fours screws that mount the Crystal Controlled Oscillator and move it out of the way (or remove it for rebuild if you've unsoldered the wires.) Now you should have easy access to T-1. On non-"X" receivers all you have to do is remove the mounting screws from the nearest filter choke and move it back and to the side to allow the cover of T-1 to be removed. Once the cover of T-1 can be removed, then you can access the single .01uf 400wvdc molded capacitor located inside. After replacement of the capacitor, the cover can be re-installed. If you are not going to rebuild the Crystal Controlled Oscillator then remount it (but if you have then resolder those wires) and then remount the filter choke-capacitor assembly and resolder those wires.

The 3.5mc Conversion Oscillator

This assembly is located under the chassis. The shield is mounted to the chassis using four 6-32 screws. Removing the screws and the shield allows access to the 3.5mc oscillator circuit. There is a vertically mounted fiber board that has some components mounted to it. There are three .022uf 400wvdc molded capacitors used in this circuit that will need to be replaced. When re-installing the shield note that there are three notches that allow the circuit wiring to pass under the shield when mounted without pinching. Be sure the wires pass through the notches. Also note that there is a notched fiber spacer that sets on top of the vertical fiber board. This must be in place when re-installing the shield since it prevents vibration movement of the vertical board that might mechanically affect the 3.500mc crystal.

The Crystal Controlled Switchable Local Oscillator ("X" option)

This unit allows the operator of an "X" optioned SP-600 receiver to substitute a crystal LO for the regular LO for increased stability. Six crystal sockets allowed the user to install HC-6 crystals for fixed frequency operation. A six position switch allowed selecting any of the crystals installed. There is also an ON-OFF function (labeled "VFO") that routes the B+ from the standard LO to the JX oscillator. A vernier frequency control was also provided. The VFO/crystal channel switch and the frequency vernier were brought out as two extra controls added to the upper right section of the front panel.

When selecting a Crystal Controlled LO frequency, only the LO is changed - you will still manually have to tune the receiver to the selected operating frequency in order to have the RF and Mixer stages also tuned correctly. You'll need to install the correct frequency HC-6 crystal which is not the same frequency that you want to tune in. The manuals will provide the formula to calculate the crystal frequency required for the desired receive frequency. Basically, 455kc or 3.955mc is added to the desired receive frequency to obtain the correct crystal frequency depending on the band and whether the receiver is operating single or double conversion. Also, on the higher frequency bands the result is divided by 3 or 4 for operation on the crystal frequency harmonic.

Removing the Crystal Controlled Switchable LO unit requires removing the knobs and any shaft couplers. Remove the filter choke/filter capacitor sub-chassis to access the wiring harness connections. Five wires have to be unsoldered there. There is also a connection to the switch and the RF Platform to unsolder. Remove the screws from under the chassis that mount the hex stand-offs that are attached to the bottom of the unit.  You should now be able to extract the entire unit from the receiver. Several screws need to be removed (and the sheet metal cover sides slightly bent outwards to remove it) in order to gain access to the Crystal Oscillator. Once inside you'll find two molded capacitors that need to be replaced. Check the resistors for drift and clean the switch to complete the rebuild. Re-assemble and re-install in the reverse order.

Once the Crystal Oscillator unit back in the receiver and is operational, it's doubtful this particular option will ever be used unless you want to listen on one frequency - exactly. When the SP-600 is running on its own LO it doesn't drift after a 10-15 minute warm-up, so only the most demanding frequency-stable military data-type (RTTY usually) requirements would benefit from using the Crystal Controlled LO.


photo above:  Inside the Crystal Controlled LO unit. Note the two orange-drop replacement capacitors behind the RF chokes. (SP-600 JX-21) 

Completion of the Electronic Rework

Under the Chassis - There are still more molded capacitors located under the chassis but they are easy to see and most are easily accessed for replacement. There are usually a couple of "bath-tub" dual capacitors that have two .05uf 600wvdc capacitors internally mounted. It's easiest to just replace these with individual "Orange Drop" capacitors. These "bath-tubs" are PCB oil-filled and present possible health issues when rebuilding is attempted. It's best to just replace these dual .05uf "bath-tub" capacitors with modern types.

Sometimes one of the .05uf dual capacitors is used for the AC line filter but dual .01uf capacitors were also used on some later receivers. Also, there are two black (sometimes silver) tubular capacitors mounted against the front of the chassis on stand-offs. These are .25uf for the AVC delay in CW mode and .05uf for the Noise Filter circuit. These can be replaced with appropriate "Orange Drop" capacitors.

Resistors -  It's a good idea to check ALL of the resistors and replace as necessary. It's common to find a few, if not several, burned resistors in an un-rebuilt SP-600. The molded capacitors have been leaky for decades and any operation of the receiver for extended periods of time will end up overheating many of the resistors. The leaky capacitors have caused the resistor failures - there is nothing wrong with the original resistor wattage ratings. I have actually found 2W resistors installed in place of burned 1/2W resistors while the "leaky" molded capacitor that caused the original problem was left in the circuit. The receiver, when operating correctly, is designed to work with the specified resistors and there is no need to "out think" the radio engineer by installing higher rated parts.  

Rebuilding the Electrolytic Capacitors - The SP-600 uses a triple 20uf electrolytic B+ filter capacitor with a built-in resistor connected between the negative terminal and the capacitor's metal case. The way in which this electrolytic capacitor is connected, with negative connected to chassis, eliminates the need for this particular internal component (it isn't shown on the schematics either.) These were extremely high quality electrolytic capacitors and many are still fully functional today. If you want to use the originals, then check them carefully for leakage and then reform them if the receiver has not been powered up for several years. Also, most SP-600 receivers have several 10uf electrolytic capacitors that are mounted in "bath-tub" containers. Again, these were very high quality units and many are fully functional BUT check them carefully. If you feel uncomfortable using 50 year old electrolytic capacitors (and maybe you should,) just replace them. I usually install the new electrolytics inside the original housings since it results in a much better under-chassis appearance.

The B+ filter capacitor rebuild involves cutting the can of this triple 20uf electrolytic to remove the old capacitors inside. Mark the can before cutting to assure it goes back together in alignment. Cut slightly above the base crimp. Once the cap is in two pieces use a heat gun to get the black tar hot enough that the old insides just slide out of the can. The same treatment will remove any black tar in the bottom piece that has the bakelite terminal base. Wear heavy gloves when using the heat gun. Once the can is cleaned out you can install three new 22uf 450wvdc capacitors with the negative leads tied together. You will have to stack the caps as seen in the photo to the right for everything to fit into the can. Use sleeving to insulate the leads and electrician's tape to hold the three caps in alignment. You'll have to drill four small holes (.060") in the bakelite base of the capacitor near the terminal posts to allow the new capacitor's leads to be connected to the proper terminals. You'll notice that the terminals that are on the inside would make great connections but unfortunately they are made out of aluminum and can't be soldered to. Don't drill through the aluminum as it is so soft it will clog the drill flutes and break the drill bit - drill through the bakelite. Wrap the new capacitor's leads around the proper terminals and solder. Now you'll have to join the top of the can back to the base. Use some paper card stock to act as a backing for the epoxy that is used to glue the can back together. Coat both sides of the card stock with lots of epoxy and fit the top back in place making sure that the marks on the can are in alignment. Wipe the excess epoxy off the outside of the can. Wrap the can with masking tape to hold the two pieces in alignment until the epoxy sets up. Paint the glue joint with silver paint to hide the seam. This isn't a perfect, undetectable rebuild of the B+ filter cap but it makes the installation of new electrolytics much neater.

The tub mount 10uf  electrolytics do not use PCB oil but are filled with the old black tar so rebuilding is easy. An old style, really large, high wattage soldering iron with the huge one-inch wide size tip is required. These old behemoths transfer enough heat to remove the bottom plate by melting the solder and prying up the bottom plate, one side at a time. Once the plate is off, dig out the tar and the old capacitor and replace with a new electrolytic as shown in the lower photo right. It's not necessary to replace the bottom plate. This process is next to impossible without using a very large soldering iron. If you don't have one, just replace the old tubs with new electrolyics installed on new tie strips for the bias supply and at the existing tie point for the driver tube and at the 6V6 tube socket for its cathode by-pass.

Wiring Harness and Soldering - Be sure to check over the condition of the wiring insulation under the chassis. The wire insulation on early SP-600 receivers is rubber-like and covered with cloth. It seems that the cloth, which probably has fiberglass in it, is prone to fraying and disintegrating depending on the amount of rework and the storage conditions that the receiver has gone through in the past. Later SP-600s use wire that doesn't have the fiberglass strands and is more like standard hook-up wire. Also, check all of the soldering rework that was done in the past. The quality of the rework can vary dramatically from receiver to receiver. Resolder any suspicious looking joints. Sometimes you will even have to strip and tin new leads on the wires that have had too much stress and breakage during the past reworks. Tubes & Tube Shields - It's obvious that all of the vacuum tubes need to be tested and replaced as necessary. Weak or marginal tubes will compromise the receiver's tremendous performance capabilities. However, don't rely entirely on the tube tester. Proper operation of the receiver is the final test for the tubes. Although uncommon, you may find tubes that test fine in the tube tester that are noisy or might have problems that only show up after 10 or 15 minutes of operation. Clean all tube sockets with De-Oxit as you re-install the tubes. The LO tube, a 6C4, has a special tube shield that has a grounding extension that is connected to the front ground binding post terminal on the top of the tuning condenser shield. The other tube shields are standard nickel plated types made by various manufacturers. The heat conducting black IERC type tube shields can be fitted to the SP-600 tubes and they do lower the tube temperature which results in longer tube life. Many SP-600 receivers were fitted with the IERC tube shields during their active use with military and commercial users. The Hammarlund manuals always show the old nickel plated, high heat-retaining types of tube shields because they were the original type supplied with the receiver.
BFO Repairs on the "JX" Versions - The BFO usually doesn't have any serious problems but if anything does develop, getting into the BFO can is a real headache if the receiver has the "JX" option. Of course, the time to thoroughly check out the BFO is when the receiver is disassembled for the rebuild. But if a problem develops afterward, you'll find it's impossible to easily access the BFO coils and circuitry inside the can because the Selectable Crystal Controlled Oscillator has a switch shaft that is directly over the BFO can. Even if you remove the shaft, you still can't get the BFO can off. To gain access to the BFO coils and other components for repairs first remove the side panel of the receiver. Next, under the chassis, unsolder the four wires that connect to the filter capacitor and filter chokes. Now, remove the screws that hold the filter choke platform to the chassis standoffs and remove the assembly. Now, remove the four screws that thread into the standoffs that mount the Crystal Controlled Oscillator and move the unit back out of the way. It isn't necessary to unsolder any of the wiring to the Crystal Controlled Oscillator. You can now remove the BFO can and proceed with any repairs. When the BFO repairs are complete, then reassemble the receiver enough to allow power up and testing. If the BFO functions correctly then complete reassembly.

Shown in the photo to the left is the BFO from the JX-21 receiver. I had noticed that the BFO wouldn't hold alignment and many times when turning the BFO knob, the BFO frequency wouldn't change. As can be seen in the photo, the metal end cap that has the threaded rod passing through it had become detached from the coil form. This allowed the blade spring which provides an end load on the slug adjustment to become fully extended. This resulted in erratic movement of the slug and BFO frequency. The repair was to reattach the end cap using epoxy making sure that the end cap was fully seated onto the coil form and held in place while the epoxy cured.

On "non-X" optioned receivers, working on the BFO is easy only requiring removal of the side panel for good access and visibility. Then removing the BFO can to observe, repair and test.

photo left: The BFO from the JX-21 receiver showing the detached end-cap and that the blade spring load is fully extended.

Major Modifications - The SP-600 was an expensive receiver that was normally used either by the military or by commercial users. It's performance is legendary. It isn't usually found with extensive modifications installed. However, most SP-600 receivers have gone from former owners that didn't usually install undocumented modifications to new owners - hams. Some hams still seem to relish the idea that they can "out-engineer" the Hammarlund engineers. Most of us know from experience that extensive ham modifications almost never function better than the original circuit that was designed by a team of professional radio engineers. Almost all ham "major" modifications are poorly installed. Almost all ham modifications tend to enhance one area of performance at the expense of performance in another area. Additionally, ham modifications are rarely documented so their design intent and expected improvement is usually a mystery.

I always try to return the receiver circuitry to the original design and layout. I always try to use components that are the same values and ratings as the original. It is acceptable to replace a 10% resistor with a 5% - but not vise versa. Replacing the molded capacitors is the exception and, when replacing the molded caps, be sure to use all the same type of new capacitors - it looks like a professional rework job then. I have found through restoring many communications receivers that the original design works best and many functional anomalies can be avoided by adhering to the original component type, original placement and original wiring layout. Once the receiver is returned to original, now all of the original documentation - no matter where it comes from - can be used and will be accurate for all future users of the receiver. Additionally, the manual's operational instructions will be accurate and the receiver will perform as described in the manual.

Major Audio Modifications - The Argument Against 'em - The audio output on the SP-600 is communications-grade with the bass rolled off at about 3db down at 125 hz resulting in a noticeable lack of bass response. Just how much this absence of bass response bothers the user is certainly subjective but Hammarlund engineers had good reasons for the original audio response shaping. The SP-600 was designed as a communications receiver and as such it must perform that function equally well in all modes of reception. While it might be pleasing to listen to an AM BC station with a bass response that goes subterranean, this type of response is a hindrance to good CW copy. Additionally, RTTY and other data modes are adversely affected with an audio response that over-emphasizes the bass. Certainly SSB communications was developing while the SP-600 was in production and, while the demodulation of a SSB signal was probably not a major consideration during the design stages, SSB intelligibility is certainly better without significant bass enhancement. The SP-600 owner should be aware that AM reception was just one of many communication modes that the Hammarlund engineers had to consider while designing the receiver.

A minor reshaping of the audio can be accomplished without limiting the SP-600's ability to demodulate CW and SSB signals. Paralleling the two 5100pf coupling capacitors in the audio section with .01uf capacitors then paralleling the audio output tube's cathode bypass with a 10uf electrolytic and adding a 10uf electrolytic in parallel with the 1st AF Amplifier's 1K cathode resistor will give the audio output a response that sounds like a typical, older communications receiver. This simple approach will satisfy most users that feel that the original roll-off of the bass was excessive and it doesn't affect the ability of the SP-600 to function normally in the CW and SSB modes.

Mechanical Restoration Work

Body and Panel Work - Many SP-600 receivers have been roughly handled during the last few decades of moving and storage. Consequently, you might find bent panels, chassis or broken parts in general. Most of the straightening of panels is common sense and routine. It normally involves removing the bent part and using either auto body techniques or using a non-marring hammer to straighten them. More sheet metal information below in the Cosmetic Restoration section.

Install ALL of the Screws - The SP-600 uses a lot of screws to mount various shields, panels and other assemblies. All of these screws are important for mechanical stability and for shield electrical connections. It is very common to find an SP-600 missing many of the screws for these parts. Most "hamsters" think that only two screws are required to mount panels (one screw, if it's really tight!) ALL of the screws are necessary for the receiver to have its mechanical strength and stability, so be sure to install all of the screws and replace any that are missing.

 Tuning Mechanism - It's common to find the tuning mechanism dirty. Remove the front panel along with the main tuning dial, the logging dial and the band indicator dial. Spray the brass gears with WD-40 and work the dirt out with an acid brush. WD-40 is cheap and a good de-greaser. When clean, lubricate only the bearings - sparingly. The brass gears really don't need lubrication and too much oil or grease are just "dust traps" that end up causing wear and other problems. Reinstall the dials and check the tuning action. It should be super-smooth. After cleaning, if the tuning slips, which is not uncommon, the problem is usually the "S" tensioning spring that loads the drive wheel against the rim of the logging dial. The "S" spring sometimes looses its "springiness" over time but you can remove the "S" spring and bend it outwards to increase the load tension when it is re-installed. Sometimes the "S" spring has just "had it" and it needs to be replaced with a new one (or a good one from a parts set.) Additionally, the contacts areas where the drive wheel engages the reduction wheel and where it engages the logging dial rim must be super clean. When everything is "right" with the tuning mechanism its operation is better than "velvet smooth" and there will be no slipping of the dial drive.
The Antenna Input Twin-ax Connector -  Most of the SP-600 receivers were equipped with a UG-103/U twin-ax Antenna Input connector mounted on top of the RF Platform. The mating connector, the PL-102 (identified as UG-102 in the SP-600 manuals,) is not something that every ham had lying around in their junk box. Consequently, many SP-600s have had their UG-103/U replaced with a standard (read - "easy to find") SO-239 UHF type coax connector. The original SP-600 design concept was to use RG-22 balanced coax (twin-ax) for dipole antennas and since this was 95 ohm Z line, the match was fairly close. Nowadays, few (if any) hams run twin-ax because the losses are so high. So, if you are going to utilize your ham station transmitting antenna for the receiver antenna, you'll end up with standard unbalanced coax running to the SP-600. You'll have to ground one of the twin pins in the PL-102 connector for proper unbalanced operation. It doesn't matter which pin is grounded, just so one of them is. This can be done by soldering a wire from the coax shield up to one of the twin pins. I don't advocate replacing an original UG-103/U connector but, if it's already replaced, once the mating coax connector is hooked up, you can't really tell if an SO-239 has been substituted for the original twin-ax connector. Additionally, the right angle UHF adapters are a lot easier to find than the twin-ax right angle adapter, the UG-104/U. If you use small diameter coax, like RG-58 or RG-8X, you don't really need the right angle adapter since the coax is flexible enough to make the bend to allow it to exit out the "antenna hole" in the original cabinet. If you are rack mounted or without a cabinet then there will be no problem with the antenna coax at all.

NOTE on UG-103/U versus SO-239:  I received an e-mail from Hammarlund SP-600 historian Les Locklear that provided information that Hammarlund approved the substitution of a SO-239 for the UG-103/U in a revision sheet dated September 13, 1951. SO-239 receptacles appear on receivers as early as the SP-600 JX-8 versions. The conclusion is that after 1951, it is possible to find a SP-600 with a SO-239 antenna receptacle that is an original installation.

photo above: The original antenna connectors for the SP-600. The UG-103/U twin-ax receptacle and the PL-102 (UG-102) twin-ax mating plug.

IF/RF Alignment

The IF Section - The IF is straight forward in alignment except for the second conversion frequency adjustment of 3.955mc. This requires the 455kc IF to have been very accurately aligned since the conversion oscillator operates with a 3.500mc crystal. Center the IF using the Crystal Filter crystal frequency and you'll end up very, very close. Be careful on the 3.955mc adjustment of T-1. It is possible to adjust this to a harmonic and everything seems okay - until you try to operate the receiver above 7.4mc and the instability lets you know something is wrong. Also, the Crystal Filter adjustment of T-3 can be adjusted to a harmonic with the result that the Crystal Filter won't resonate correctly. This is a common problem on some SP-600 receivers. Check the height of the adjustment screws on all of the IF adjustments and be suspicious of any adjustments that "peak" when they are at the limits of either end of adjustment.

 

Crystal Filter - This circuit seems to have a lot of critics that claim it doesn't function very well, if at all. Most of the problems with the Crystal Filter operation can be traced to either poor alignment of the IF section or improper placement of the Phasing knob in relation to the phasing condenser's position. The IF alignment frequency must be determined with the Crystal Filter set to the sharpest position. It will probably not be exactly 455 kc but will be very close and this is the frequency that all of the single conversion IF transformers are adjusted to. Align the IF with the selectivity in the 3kc position. To set the Phasing knob's position, inject an unmodulated 455kc signal into the 1st Mixer stage. Don't over-drive the IF with a strong signal. Switch to CW and tune the BFO for about 1kc audio output, set the selectivity to 0.5kc and then rotate the Phasing control slowly. When rotating the Phasing knob, you'll notice that the selectivity varies from wide to very narrow and near the narrowest position you'll eliminate the 1kc tone. Set the Phasing knob so that the widest bandwidth is at 0 and the narrowest is at 10. Elimination of heterodynes will occur around 8 or so on the Phasing scale. The "diamond" mark on the Phasing scale indicates mid-range on the selectivity.

The RF Section - The RF adjustments are accessed through eight holes that are located on the RF Platform. Each of the holes is covered with a 3/8" diameter hole plug that has to be removed - they just snap in place and are easy to remove. A long alignment tool is necessary to reach the adjustments that are actually on the turret band switching assembly. The alignment tool should be non-metallic to avoid de-tuning the LO on the higher frequencies. Most of time I need a small flashlight to peer down the alignment hole and see the L or C adjustment screw so that I'm not poking around trying to "feel" when the alignment tool engages the adjustment screw. It is possible to actually engage one of the slots in the tensioners and then loosen it up so the screw adjustment isn't capable of holding a setting. Better to look and make sure of proper engagement of the adjusting tool. A 100 ohm resistor has to be used in series with the RF Signal Generator so the load Z remains higher than 100 ohms. This allows the 1RF amplifier stage to be set for the 95 ohm twin-ax but it is also close enough for matching up with most antenna loads. Remember, the SP-600 doesn't have an Antenna Trimmer, so you have to be careful in presenting the right Z load to the receiver input. The SP-600 alignment instructions tell you to start with the lower end of the tuning range and adjust the L but I have found that this method results in a lot of readjustment back and forth between the upper and lower ends of the range. It's better to start at the high end of the range and adjust the C and then proceed to the lower end and adjust the L. This usually only requires two back and forth adjustments.

What is really nice about aligning the SP-600 is that all of the adjustments are done from the top of the chassis - no need to have the receiver on its side and have to look at the tuning dial sideways.

Cosmetic Restoration of the SP-600

Front Panels - The front panel of the SP-600 is pretty easy to remove and it should be stripped of the dial escutcheons, the plastic window for the band switch dial and the meter so that you have just the panel itself. Cleaning can be done with dish soap and water or any other sort of mild cleaner. I avoid using Windex because it has ammonia as its active cleaner which can remove some kinds of paint used for panel nomenclature. Glass Plus which is ammonia free seems to work very well. Most panels will have some chipping that can be touched up using matched paint. The panel can then be lightly polished or waxed if desired since the original paint is super tough.

Repainting the Front Panel - This should only be done if the panel is really in poor shape and doesn't look very good after cleaning and polishing. First, if you want to maintain originality, have the front panel paint matched at an automotive paint shop. This way you end up with high quality paint that's the correct color. Many automotive paint stores will fill spray cans with the matched paint for a nominal fee. This makes painting the panel much easier. One can is usually more than enough paint to do a single front panel about three or four times. You probably will have to spray the panel at least twice since usually the first try will have too much paint applied.

The original paint is very, very tough and will require a couple (at least) applications of heavy-duty epoxy stripper. Make sure the panel is super clean and degreased. Original SP-600 panels look like they didn't use a primer and since you are going to have to fill the engraving you don't want to have anything that is going to add more paint than is necessary. Spray a couple of coats and see how the engraving looks. If it's still pretty sharply defined and the panel coverage looks good then stop. Too much paint will fill the engraving and prevent proper nomenclature filling. Two coats should be the maximum. Let this dry for at least a day or two. Most automotive paints will "shrink" somewhat while drying. At first the nomenclature will look like there is too much paint applied but wait a day and see how the paint is after it has fully dried.

Nomenclature Fill - For the nomenclature fill, I mix Artists Acrylics of White, Raw Sienna and a little Black to end up with a Manila color. "Pure white" will look way, way too bright. Do only one or two sets of engraving at a time. Apply the paint with a brush into the engraving and you don't have to be real careful - just paint it in. Let the paint set for about one minute. First remove the excess paint using a squeegee. Then use a small piece of paper towel folded a couple of times that has been dampened (not dripping wet) with Glass Plus. Don't use Windex - only Glass Plus works for removing the excess fill paint without damaging the panel paint. Lightly rub the dampened paper towel in a circular motion with the paper towel absolutely flat on the panel. As the excess paint not removed by the squeegee is beginning to come off, flip the paper towel over to the clean side to continue to remove the excess paint. I always have several pieces of towel ready because you'll need at least two pieces (four sides) to remove the excess paint. Let the paint in the engraved area set for a few minutes more and then wipe the area down again with another dampened paper towel to fully clean the area  I usually have to apply some more fill paint and repeat the process to get the lettering to really look professional and proper. Let the fill paint dry for a day. Finally, use carnauba wax and apply a nice polish to the panel. It will look great.
Chassis Cleaning with Easy-Off Oven Cleaner - SP-600 receivers were normally used in controlled environments during their active lives. However, most of the receivers have now become victims of poor storage in humid areas such as garages, basements or worse - sheds. The moisture damage to the aluminum usually shows up as oxidation and dullness to the finish or, worst case, pits. This kind of damage can be somewhat removed by using Sodium Hydroxide. A convenient way to apply Sodium Hydroxide is with Easy-Off Oven Cleaner (EOOC.)

 Spray a small amount of EOOC into a cup and using an acid brush or Q-Tip, apply a coating to the chassis. EOOC does react with aluminum but you aren't going to leave it on the surface for more than just a few minutes. Wipe off the EOOC and then swamp the area with water using either a brush or Q-Tip. Gently wipe and repeat the water rinse a few times to neutralize the EOOC. Application of the EOOC should be performed in a well-ventilated area. The surface aluminum will be left very clean AND will have the flat finish of the original aluminum. This treatment leaves the aluminum very clean but doesn't "polish" or scratch to remove the oxides and dirt.

Nobody wants a "polished to a mirror" finish on a receiver chassis. Most restorers want the aluminum chassis to appear as it did when the receiver was fairly new. The EOOC treatment ends up with the aluminum looking "flat" original. The IF transformer shields and all of the the other removable aluminum pieces including the side gussets can be taken outside and, one at a time, sprayed with EOOC, left for three minutes and then rinsed with water from a hose. This ends up with the aluminum pieces looking very clean and with a flat finish like they had when they were new. Be sure to rinse all the pieces thoroughly with water. Also, don't rub the aluminum pieces with anything - just let them air dry. Rubbing will actually end up slightly polishing the aluminum piece and will ruin the flat finish you are trying to achieve. When cleaning the IF, BFO & T-1 shields, be careful of the lettering on top of the cans. The lettering is very delicate and almost any cleaner seems to destroy it. Avoiding the lettering works best. This EOOC trick only works well on aluminum. It doesn't have too much of an effect on steel - it cleans (as it's supposed to clean your oven) but that's all.

Components like the power transformer and the chokes can be cleaned with any mild cleaner then touched-up as necessary. If they look a little dull, they can be given a coat of Armor-All.

Tuning, Logging and Band Indicator Dials - There are three dials on the SP-600, the main tuning dial, the logging dial and the small band indicator dial. The paint on these looks durable but it's common to find the paint cracked and flaking off. Also, staining from excessive moisture encountered during poor storage conditions is common. You'll have to test a small area of the dial paint to see how well it is adhering to the dial base. I use a small paint brush and gently brush the area and look to see if I see flaking. This is a sign that the dial paint is beginning to crack due to surface corrosion on the dial base. If you don't see any residue coming off with gentle brushing then the dial paint is in good shape and can be cleaned very gently with a damp cotton cloth. Some dials will be found with the paint already falling off and with surface corrosion apparent in the areas missing paint.

It appears that the original method used to finish the dials was to chemically apply a black oxide coating to the dial. This might have been black anodizing but it could also just be an oxide treatment. Next, the white paint was applied into the areas that are to be the background color. This had to be a relatively slow drying paint that was allowed to settle leaving the high points - the dial numerals - slightly above the paint. Then a squeegee type of device skimmed the high points to reveal the black oxide underneath.

I haven't tried this method of dial restoration since spare original dials are so expensive and I don't want to take a chance on ruining a decent set of dials. However, I do now have a spare set of SP-600 dials that are in poor condition and sometime in the future I'll attempt this type of restoration. For the time being though, the only accepted solution for poor condition dials is a photo paper overlay that is glued to the back of the dials and then reverse mounting the dials. These never look original and are only a "stop-gap" solution until someone develops a good restoration method for the SP-600 dials.

Dial Pointers - The dial pointers have a black oxide finish on them. Sometimes they have scratches but touch up is easy with a black marker pen, like a "Sharpie," which gives the sort of "transparent" look that the original finish had. The main tuning dial pointer can be removed with two small screws while the upper pointers can be removed by dismounting the dial lamp assemblies.

Escutcheons and Plexiglass Screws - The escutcheons seem to take a lot of hits and are usually scratched up. Repainting is sometimes necessary but usually they will touch up fine and look more original that way. I use satin black spray paint that is sprayed into a small cup and then applied with a very small brush. The satin black seems to blend better than gloss, even though the escutcheons are somewhat glossy. If you've repainted the front panel, you'll probably want to repaint the escutcheons, too. Be sure to use high quality automotive paint as this will give the absolute best results for your repaint. The screws that hold the plexiglass dial covers always seem to be rusty. Just wire brush and paint with flat black paint. Don't torque these screws tight or you'll crack the plexiglass. Just barely snug is all that is needed. 

RF/AF Level Meter - The RF/AF Level Meter on the early SP-600 uses a paper scale that is very prone to moisture damage since the meter itself is not a sealed unit. If the scale is really stained badly, it can be removed and covered with a reproduction meter scale. You can make your own repro scale if you can scan the stained original scale into a computer. Disassemble the meter and remove the scale. Scan the scale into the computer. Use a "photo editor" program to correct the defects. Sometimes the discoloration is too severe to easily remove in the photo editor. In that case you can try making successively lighter copies (on a copy machine) of the scale until the nomenclature remains but the color background is even. Then scan that copy into the computer and photo edit out any remaining defects. Size it correctly and make the final print out on photo paper that is non-gloss finish. I specify photo paper for a fine texture finish and clean printing. The final scale copy has to be lightly glued over the original scale backing. When the meter is reassembled make sure the needle doesn't drag on the new scale since it is now a little thicker. Also, clean the needle stops of corrosion (common problem.) When the entire meter is together it is very difficult to tell that the meter scale is not original - except that it looks a lot better than most originals do. The later Marion Electric meters are much higher quality and use a paint-on-metal meter scale.

photos left: Before and after showing the early RF/AF Level Meter scale restoration. Staining of the paper scale can be seen in the before photo (SP-600 JX-21).

Cabinets - Most SP-600s were rack mounted, usually in multiple receiver set-ups. Originally, the SP-600 could be ordered as a "table mounted" receiver with cabinet. The table mounted receivers did not have bottom covers since the cabinet provided the same function. It was possible to order a conversion kit to convert a table receiver to a rack mount. Although it is mentioned in the manual, it seems likely that someone could have ordered a cabinet from Hammarlund to convert a rack mount to a table mount. Since most receiver were in racks, there are more receivers around than original Hammarlund table cabinets and this has resulted in the original cabinets becoming fairly high priced. Since the SP-600 has a very deep chassis only a few types of non-Hammarlund cabinets are deep enough to house the receiver. The original cabinets do not have threaded holes for mounting the panel of the receiver to the cabinet. Instead, a drilled and tapped metal strap fits behind the panel mounting flange and then the panel screws thread into the strap. There are also a couple of holes in the bottom of the cabinet to allow securing the back of the chassis using the tapped mounting brackets that originally hold the bottom panel on. Cabinets are usually wrinkle finished and can be found in several different shades of gray ranging from light grayish-brown to dark charcoal gray. The cabinets can be cleaned with Glass Plus and a soft brass brush, like a suede brush. Soak each surface in turn with the Glass Plus and then work the brass brush gently in a circular motion to work the dirt and grime out of the wrinkle convolutions. Wipe the area with a paper towel and repeat until the paper towel wipes are fairly clean. Don't scrub too long as the paper towels will never be perfectly clean and you are really seeing some "dead" paint as color on the towels. Usually two applications is enough. Let the cabinet dry thoroughly and then do any touch-up that is necessary and let that dry. Finally, apply either Armor-All or Boiled Linseed Oil (not both) and wipe off the excess. The cabinet will look beautiful.

SP-600 Performance Expectations

 The SP-600 performance is legendary. However, many hams who have heard or used an SP-600 in the past were exposed to a receiver that was not operating at its full capabilities. The receiver was designed to somewhat function will several defective parts, weak tubes and poor alignment. A newly rebuilt and freshly aligned SP-600 returns the receiver back to operation that is at or better than the original specifications. When everything is working correctly on an SP-600, you'll find it's a fabulous receiver that is a pleasure to operate and even more of a pleasure to listen to. The large knobs impart a great feel to the operation and the ultra-smooth tuning adds to the enjoyment of searching for stations. The sensitivity of a properly aligned SP-600 is impressive and, with a matched antenna, you'll hear everything that is on frequency with conditions or QRM being the only limitation.

The selectivity is very good at the 3kc position for SSB and CW signals. AM signals can have the bandwidth expanded to 8kc and if the station is very strong, try 13kc for the ultimate in wide bandwidth audio response. The Crystal Filter has it critics but I find that it works very well on CW but even SSB or AM can benefit from its use when adjacent frequency interference is a problem. Accurate alignment of the crystal filter is crucial for its proper operation. Selectivity is very sharp in the crystal filter position and you can also vary the phasing from center to reduce heterodynes or peak CW signals.

Many users (or maybe potential users) feel that the tuning dial's vague readout accuracy is the main drawback to the receiver and generally you'll find these critics comparing the SP-600 to the R-390A. These two receiver's utilize design approaches that couldn't be more diverse - not to mention that the R-390A cost was at least two times that of the SP-600. The SP-600 covers large slices of the spectrum in each tuning range. You just couldn't get one kilocycle accuracy when each tuning range covered several megacycles. The only way Collins could get 1.0kc accuracy was to limit the coverage of each tuning range to one megacycle. If you want a 30 band receiver, you can have the 1.0kc accuracy. The SP-600 allows rapid coverage of wide areas of the spectrum - something necessary for certain kinds of surveillance.

 

photo above: This 1953 SP-600-25C was purchased at a local ham swap meet for $50. The receiver's nonfunctionability accounted for its very reasonable price. I've fully rebuilt the receiver internally but cosmetically it's still pretty much in original condition, including the excellent condition original tuning and logging dials. I did have to replace the Crystal Filter Phasing knob because of a broken section on the bakelite part of the knob, otherwise the cosmetics are original. The SP-600-25C receivers were built for the Signal Corps and have a very large 25-60 cycle power transformer and don't have the switchable crystal controlled oscillator option. The "CAUTION" tag in the upper left corner of the panel is not original but,... hey, it fits. This receiver was probably designated R-483 by the SC. This original Hammarlund cabinet is medium gray wrinkle finish and is a good match for the over-all gray and black color scheme of this receiver. The cabinet was not with the receiver at the swap meet.

The logging scale was provided to allow a means of accurate resetability and its use will really help in finding net frequencies for monitoring. Once you've found the net, make a note of the logging scale reading and you'll always be able to retune right to the correct frequency. Or, you can always use a heterodyne frequency meter if you want to know exactly where you are tuned - that's the way they used to do it before digital frequency readouts.

As mentioned elsewhere, the Crystal Controlled Switchable Oscillator - the "X" option - is rarely used. It requires specific crystals for the desired tuned frequency and then you still have to tune the receiver to the desired frequency so that the RF stages and the Mixer are in tune. The typical SP-600 doesn't drift after a 10 minute warmup.

The SP-600's audio quality is highly subjective with some users finding it to be standard "communications grade" with no bass response while others listeners find the audio to sound "high fidelity." The stock bass response is rolled off so that the 3db point is at about 125hz resulting in a mild bass at best. Stronger AM stations will have some depth to the audio (depending on your speaker.) AM ham stations that are running some power sound pretty good and the retired AM BC transmitters operated on 160M or 75M will sound best. Be sure to match the 600 ohm output for the best audio reproduction and use a large speaker in a good enclosure. The audio has a lot of highs when the bandwidth is in the 13kc position. In the 13kc position you will note that the IF gain is slightly reduced, this is normal and part of the design for best audio reproduction with expanded bandwidth. Major audio modifications to greatly enhance the bass response are not recommended as this will seriously alter the receiver's capabilities for CW and SSB reproduction. Minor shaping of the audio will improve the response while still preserving the receiver's performance in modes other than AM.  >>>

 >>>  The calibrated BFO can be very accurate and very handy for SSB. You do have the capability of selecting upper or lower sideband by which side of "0" you set the BFO. Drift is just about non-existent after about a 15 minute warm-up. To set the BFO to the correct sideband, first tune in the SSB station to "zero beat" or where the audio sounds "muffled and bassy" - then, by adjusting just the BFO to one side or the other of "0," the audio will clear up and sound normal. Whichever side that is will be the correct setting for whatever sideband that station is using and more than likely for all the SSB ham stations on that particular band.

The SP-600 will provide excellent SSB audio. And you don't need a Product Detector or a CV-591-type of external SSB IF unit either. Good SSB demodulation requires that the detector has the proper ratio of BFO injection to the incoming signal level. Best audio will require that the AVC is in the OFF position and that the RF gain control used to set the correct ratio. Turn the AF gain up to 6 or 7. Turn the RF gain to a level where the incoming received background noise is moderate - usually about 5 or 6. Selectivity should be on 3kc. With an SSB be signal tuned in, lower the RF gain if the audio sounds distorted. At the proper RF gain setting, SSB signals sound great on the SP-600. The disadvantage is that every signal has a different strength and the RF gain usually has to be adjusted each time. With "one on one" QSOs, it's usually not a problem but Net Operations, with many different stations "checking in," can be somewhat frustrating. You can operate with the AVC on to limit the maximum sensitivity (with the RF gain set to about 7) but strong SSB stations will probably still distort. Most versions of the SP-600 provide an adjustable BFO Injection adjustment located on the rear apron of the receiver. This can be adjusted to a higher level if you plan on mostly SSB reception rather than CW, although the highest level is still a lower injection level than what is required for SSB demodulation with high level signals. Low-level BFO injection was normally used for CW signals and prevented "masking" of very weak CW signals. Some modifications will increase the BFO coupling capacitor value to increase the total injection level at the detector but this is really unnecessary since it's much easier just to reduce the RF gain level.  >>>

By using the ham station antenna you will be providing the SP-600 with a matched antenna that will allow it to perform at its best. Many casual listeners using unmatched end-fed wires may be disappointed at the SP-600s performance, especially when considering the high noise levels experienced with end-fed wires. The SP-600 was designed to operate with a matched antenna and this set up will give the best performance results.

Most SP-600 versions will have a RELAY socket that looks like an AC receptacle. This parallels the SEND-REC switch and allows the receiver to be put into stand-by remotely. This can be done with the auxiliary contacts on a Dow-Key type relay and it makes using the SP-600 as the station receiver very practical. Be sure to leave the SEND-REC switch in the SEND position when using the remote stand-by function. An exception to this RELAY socket is the popular SP-600 JX-17, which, since it was for diversity reception, does not include a remote standby option.

While the SP-600 may not provide the dial accuracy of an R-390A, you won't strain your wrist tuning in stations while band cruising. You'll hear the same stations that the R-390A would receive and they may even sound a little bit better.

photo left: 1953 SP-600 JX21. I've owned this receiver for many years and it has undergone several restorations during my ownership. At one time I even painted the front panel green (see photo below.) I recently found an SP-600 "parts chassis" that provided a mint condition set of dials, a good working Selectivity switch and many other minor parts that allowed a proper restoration back to "stock" appearance along with great performance. I had the front panel paint matched from the SP-600-25C receiver and it is an automotive type acrylic enamel paint. I also returned the cabinet color to the standard gray  - it was black.

 

Collectors Photo Gallery of SP-600 Receivers

SP-600 JX from the RCA David Sarnoff Research Center

This incredible SP-600 is owned by Jeff James W2NBC. It was purchased by RCA's David Sarnoff Research Center in 1957. Since the panel is RCA Umber in color and the cabinet is also the grayish-brown that RCA used in their equipment, it seems likely that this receiver was part of a special order from Hammarlund and painted in the RCA colors. The RCA "meatball" logo is fifties vintage but the tag in the upper left corner uses the later "block" RCA letters (came about in 1964.)  There was also a calibration tag above the RCA meatball that was last dated in the sixties. Jeff has restored this special SP-600 but the panel is original paint. The cabinet was matched to the original color found under the grab handles.

SP-600 JX-14

SP-600 JX-14 with a light gray panel and black nomenclature. Photo from eBay.

SP-600 JX

Though this SP-600 is in fairly rough condition one can't help noticing that the front panel has been painted light blue and the dial escutcheons painted dark blue. Photo from eBay

SP-600 JX-17

SP-600 JX-17, standard configuration. The JX-17 was probably the most produced version of the SP-600. It was designed specifically for diversity operation and has a slightly different AVC system because of that. The red knobs are only found on the JX-17 version. Photo from eBay.

SP-600 JX-21

Here's a photo of a JX-21 with a front panel that I painted Hammarlund green. The color was matched to an old BC-1004 (SP-200) receiver so there would be some connection to an actual Hammarlund paint color. Although the receiver looked great and worked fine, a couple of museum visitors actually asked if the SP-600 was a Heathkit product - I was flabbergasted! After that, I had to return the panel to original Hammarlund SP-600 gray. The appearance now of this SP-600 JX-21 can be seen in the photo in the "Performance Expectations" section above and in the header photo at the beginning of this article. The cabinet looked like an original black paint job but after removing one of the rubber feet, I noted it was a repaint and the original color had been gray. I have also returned the cabinet back to its original color.

SP-600-JX & R-274B (SP-600-JX-6)

I picked up these two SP-600 receivers from N7RCA this year, 2013. They are both early versions of the SP-600. 

The SP-600-JX (upper unit in photo) is the victim of a hamster rebuild where "junk box capacitors" have been installed. That's where none of the capacitors match in manufacturer, type or specification - it's whatever you had in the "junk box." Also, this receiver has a replacement audio output transformer installed with purple plastic wire used - another sure indication of a "hack job." However, the dials on this receiver are excellent and most of the cosmetics are pretty good except for the hole plug in the front panel and the incorrect knobs on the Crystal Oscillator Selector. Also the Xtal Freq logging tag is missing.

The R-274B hasn't faired as well cosmetically with a poor condition logging dial and a severely scratched front panel. Also the cover for the bottom of the RF tuner box is missing which means the turret was exposed for some time which may have resulted in some damage. There is some corrosion on the aluminum parts. The circuitry appears to be all original. This R-274B still has all of the Sprague Black Beauties installed. This receiver is identified on its Hammarlund tag as SP-600 JX-6.

As I proceed with the restoration of these two SP-600 models I will add info as needed to the restoration text. I'll also detail what rework needed to be performed and how the receivers end-up performing. Both of these SP-600s are typical of fairly good "as found" condition receivers that need a lot of work but are mostly complete and very restorable.

Conclusion

More Information Wanted:   We always try to present the most accurate information available. We depend on Hammarlund SP-600 collectors, restorers and fans to provide information or corrections for this article. This results in accurate information that helps all enthusiasts in the preservation and operation these marvelous receivers. We are looking for the following information.

1. Want to add your SP-600 rebuilding experiences, ideas or photos? I always give full credit for your contributions if I incorporate your info into this webpage article. Performance thoughts?

2. If you have an original SP-600 that has a panel painted some color other than Hammarlund gray, send a photo. I would like to have better confirmation that other panel colors can be found and that they were original. I've seen original (or at least vintage) green and khaki but don't have photos.

For Sending You SP-600 Photos or Other Information - Here's the e-mail link: WHRM - SP600 INFO

 

If you want to know more about how the SP-600 came into being by reading about its "older brothers" (the Hammarlund SP-600  predecessors,) read our article "The Incredible Pre-war 'Super Pro'" which details each of the early Hammarlund models - the Comet Pro, and all of the pre-war Super Pros, the SP-10, the SP-100 and the SP-200  receivers, along with the WWII Military versions, the Power Supplies and lots of other Hammarlund information. Navigation link below.

 

 

Henry Rogers WA7YBS © December 2009, more info added January 2010, more info added August 2010, re-edit June 2011

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